The present invention relates to a controllable camshaft for a drive preferably an internal combustion engine.
In a four stroke internal combustion engine, the shapes of the cams actuating the engine inlet and exhaust valves determine the timing of the intake and exhaust valves relative to the top dead center of the piston movements. The particular cam shapes, therefore, affect the engine output torque gradient, instantaneous engine speed, idling, fuel consumption, and exhaust emissions of the engine. In addition to the opening times, the shapes of the cams also determine the time the valves remain open, which likewise affects operation. With narrow intake cams, the intake valves can be controlled to ensure optimum mixture delivery at low engine speed. On the other hand, better engine output is achieved at higher speeds with a wider (peak) cam, but a torque loss results at lower speeds.
Control of the timing of the engine valves also influences the exhaust gas emissions. The sooner the exhaust valves open, the sooner the oxidation process in the combustion chamber is interrupted, and the larger the resulting hydrocarbons in the exhaust gas. If the exhaust valve closes earlier, or the intake valve opening time is advanced, the proportion of the residual gas in the exhaust gas increases and NO.sub.x is reduced. The closing time of the intake valves, however, has relatively little effect on exhaust gas emissions.
In previous designs, the engine conditions resulting from different control times of the intake and exhaust valves were considered separately. The overlap time of the opening of the intake and exhaust valves, however, also affects the exhaust gas make-up. A large overlap at top dead center results in rough idling and increased fuel consumption during idling, and the exhaust gases contain a higher share of hydrocarbons and carbon monoxide with lower NO.sub.x. By decreasing the overlap, idling is improved, and there is a resulting decrease in fuel consumption, hydrocarbons, and carbon monoxide. In a diesel engine, on the other hand, a large overlap in top dead center may be desired to reduce knocking at cold temperatures.
As a result of these considerations, intake and exhaust cams are normally provided with distances between peaks which constitute a compromise for the various speeds and operating loads of the engine.
German OS No. 1,924,114 discloses a camshaft for an internal combustion engine in which the width of the cam is varied in accordance with engine speed. The operating cams each consist of two partial cams, one of which is carried on an outside hollow shaft, the other is carried on a rotatable inner shaft. By pivoting the shafts relative to each other, the width of the compound cam is changed. One of the shafts is rigidly connected to the drive gear for the camshaft. A centrifugal regulator has a cresent shaped weight which with increased engine speed (and thus camshaft rpm) swivels about an eccentric center of mass on the drive gear in opposition to the force of a counter-spring. The motion of the weight is transmitted by a rod system to the other of the two shafts which is not rigidly coupled with the driving gear. This arrangement requires a complex structure, and the relative pivoting is not freely adjustable but is determined by the spring constant of the counter-spring and the mass of the centrifugal regulator weight.
German OS No. 1,947,362 discloses a camshaft in which the distance between the intake and exhaust cam peaks may be modified as a function of engine speed. The device has a pair of aligned shafts whose ends in the region of the timing drive gear are coupled by spacer segments. A centrifugal governor, having springs and a weight, is interposed between the segments to couple the segments. This arrangement necessitates a predetermined relative position of the intake and exhaust valves, and without a separate arrangement, provides only relative pivoting of the two shafts and not independent pivoting relative to top dead center of the engine piston. Also, this device is not suited for compound cams, in which the width of the cam can be modified.
In German OS No. 2,029,911, each cam is associated individually with a device for ficticious modification of the cam width as a function of the engine speed. This is obtained by a speed dependent variation of the position of a cam swivel axis eccentric relative to the shaft axis.
U.S. Pat. No. 4,332,222 (corresponding to German patent application P 28 22 147.8) discloses a camshaft arrangement containing apparatus for producing relative displacement between an inner camshaft and a hollow outer camshaft. The adjustment mechanism includes at least one cam plate which is displaceable along a radial guide on the camshaft timing gear. The cam plate is displaced as a function of an engine operating parameter, and includes at least one lateral adjustment curve which controls the position of a follower sensing lever connected with one of the two concentric shafts. A spring acts on the lever to retain the lever in contact with the cam surface. In accordance with this device, using a relatively small number of parts, it is possible to adjust the rotational position of either one or both of the inner shaft and hollow outer shaft relative to the camshaft drive gear, and effect a corresponding adjustment of the cam width or the cam separation. This arrangement permits not only the two shafts to swivel relative to one another, but also permits either or both of the shafts to swivel, and thereby the cams to be advanced or retarded, relative to the top dead center of piston movement. It is also possible in this arrangement to control the position of the inner and outer shafts responsive to more than one engine parameter, for example responsive both to instantaneous engine rpm and engine load.